Real time process control of electrophotographic devices is a desirable goal in the improvement of operation quality. One important parameter to be controlled is charging characteristics, and the levels of charge placed on the photoreceptor, or other surfaces to be charged in such devices. To control the levels of charge placed on these surfaces, it would be advantageous to continuously monitor the photoreceptor surface voltage, and adjust the charging system output continuously on the basis of the measured voltage. Differences in electrostatic forces are the motive force for toner development, and optimum control of the differences is the role of electrostatic voltmeters and control loops incorporating such measurement devices. Thus, for example, potential uses of an electrostatic voltmeter may be for voltage measurement and control of the photoreceptor charging by a corona charging device, measurement of dark decay between two points along the photoreceptor in the process direction, or measurement of the discharge potential from a standard test patch to set developer bias for optimum development. A real time voltage measurement system would desirably have a fast response time, a wide range of voltage measurement, accuracy and sensitivity within 1%, and relatively small size and be insusceptible to damage caused by high voltage electrical discharge. Such systems are also required to be relatively inexpensive, as high cost would mitigate against use of the device in smaller, lower cost devices.
In the past, the primary method of voltage sensing has been to provide a non-contacting capacitive pickup adjacent a charged surface in conjunction with a high impedance device to connect the pickup to signal processing circuitry which eventually will provide a signal to the control system of an electrophotographic device or to a desired display. It will be understood that a high impedance device is necessary to avoid discharge of the relatively high voltage on surfaces such as a photoreceptor into the sensor, which may affect operating characteristics of the reproduction device and the sensor.
Currently available electrostatic voltage sensing devices include primarily transistor amplifiers to amplify the signal from the pickup into a useful form for control purposes. These transistor amplifiers are highly susceptible to voltage breakdown. The environment about the photoreceptor is extremely hazardous to transistor use, since potentials of 1000 volts or more may be accidentally applied to the sensor through floating paper fibers, dust, etc., which would destroy the transistor circuitry.
Electro-optic materials provide variations in optical characteristics, depending on applied voltage. As demonstrated by U.S. Pat. No. 4,446,425 to Valdmanis et al., a traveling wave Pockels cell may be used to measure electrical signals applied thereto by noting variations in light passing through the Pockels cell. Pockels cells, comprising a lithium niobate or lithium tantalate, crystal provide satisfactory and detectable linear response to applied voltage, but required relatively high voltages to drive the electro-optic effect, are difficult to manufacture, as they require precise cutting in fabrication stages, and are somewhat sensitive to environment. Liquid crystal materials can provide a generally linear response to voltage changes applied to such materials. As described in U.S. Pat. No. 3,934,199 to Channin, optical changes in liquid crystals may be observed to be a function of voltage. Similarly, U.S. Pat. No. 3,627,408 to Fergason demonstrates applying an electric field to liquid crystals to produce an observable effect. U.S. Pat. No. 4,579,422 to Simoni et al teaches that the electro-optical response of liquid crystals subject to an applied voltage is very linear.
As described in Xerox Disclosure Journal Vol. 2, No. 6, November/December 1977 at page 91, variations in light transmissivity through liquid crystalline material may be used to determine changes in voltage on a charged surface. However, this disclosure fails to teach what electro-optic effects serve to provide satisfactory response times or a usable modulation mechanism which would impart voltage information onto a light beam transmitted therethrough. Because of their common usage in consumer technologies such as calculator, watch and television displays, the cost of liquid crystal materials is very low. Additionally, cells incorporating liquid crystals demonstrate relatively high impedance as seen by the photoreceptor, with relative insensitivity to instantaneous high voltage discharges therethrough.
U.S. Pat. No. 4,112,361 to Nakada et al. also shows a liquid crystal voltmeter comprising a nematic liquid crystal material providing a DAP effect sandwiched between polarizing filters and transparent electrodes. An analog voltmeter is provided by applying a voltage across the electrodes and noting variations in transmissivity of the liquid crystal cell.